And to answer the question actually in the thread title: 60 grit sandpaper.
Good one. And here I was thinking: Industrial meat slicer, set to deli-thin, and being slowly fed in, feet first.
Lock yourself up in your garage, attach a hose to your tail pipe, run it into the window – you’re all set!
The least painful, actually painless, way to kill yourself, as cmyk said, is carbon monoxide poisoning. And you don’t have to go out and buy a thing. Just wait until the end of a long day, go sit in your garage with the door closed, start your car’s engine, and just fall asleep. Sweet, sweet, painless death awaits.
…Oh, you weren’t looking for real answers?
ETA: I see Guinastasia beat me to the punch.
Least painless way? Death by a thousand cuts…with salt.
Even worse if you’re on a salt-restricted diet. 
You would probably bleed out within minutes. How about being slowly lowered into liquid nitrogen, feet first? That’s gotta hurt, but it wouldn’t be fatal for hours.
I’m not entirely sure this thread is a good idea…but just to be sure I’m closing it.